The Descent of Man

to sit in the evening to listen to a number of
little Hylae, perched on blades of grass close to
the water, which sent forth sweet chirping notes
in harmony. The various sounds are emitted
chiefly by the males during the breedingseason,
as in the case of the croaking of our
common frog. (48. Bell, 'History British Reptiles,'
1849, p. 93.) In accordance with this fact the
vocal organs of the males are more highlydeveloped
than those of the females. In some
genera the males alone are provided with sacs
which open into the larynx. (49. J. Bishop, in
'Todd's Cyclopaedia of Anatomy and Physiology,'
vol. iv. p. 1503.) For instance, in the edible
frog (Rana esculenta) "the sacs are peculiar to
the males, and become, when filled with air in
the act of croaking, large globular bladders,
standing out one on each side of the head, near
the corners of the mouth." The croak of the male
is thus rendered exceedingly powerful;
whilst that of the female is only a slight groaning
noise. (50. Bell, ibid. pp. 112-114.) In the